1. Field of the Invention
The present invention relates generally to cookware and, more particularly, to stick resistant cook surfaces and methods of making the cook surface more stick resistant and more resistant to scratching, abrasion and marring.
2. Description of the Prior Art
In vessels that are used for cooking or baking, such as pots or pans and the like, it is desirable that the vessel warms or heats evenly across its bottom and sides when placed on a stovetop burner or into an oven. It is also desirable that the inner surface of the vessel, i.e., the cook surface that comes in contact with the food being prepared, be constructed such that the food being prepared does not deposit, adhere or stick to the inner surface. Although the properties above are important, it is also vital that cooking or baking vessels be resistant to damage if dropped as well as resistant to corrosion resulting from regular use and cleaning.
The use of multi-layer composites or clad metal products of three metal layers or less is well-known in the cookware and bakeware art. For example, stainless steel cooking utensils with a copper bottom are known, as well as aluminum cored stainless steel. A particular problem with cookware made from these types of materials is an inability to get quick and even heat transfer over the entire pan.
Many attempts have been made to overcome the above-mentioned problem. U.S. Pat. Nos. 4,103,076; 4,246,045; 4,646,935 and others to Ulam disclose multi clad cooking vessels. My more recent patents, U.S. Pat. Nos. 6,267,830 and 6,109,504 to Groll, are directed to improved copper core multi-layer component cookware and griddle plate, respectively. The disclosures of the above-mentioned Ulam patents as well as my own patents are incorporated by reference herein. An example of the disclosed cooking utensils is a nine-ply material consisting of a copper core clad on each side with layers of pure and alloyed aluminum, which is, in turn, clad on both sides with stainless steel. The utensils disclosed provide improved heat distribution and stick resistance, but there remains an ever-present demand to improve stick resistance of bare metal cook surfaces as well as to increase the mar resistance and increase the life of stick resistant coatings.
In order to overcome the problem of food sticking to the cooking surface, it has become common practice to coat the cooking surface of cooking or baking vessels with non-stick materials such as amorphous fluorocarbon polymers. Examples of commercially available amorphous fluorocarbon polymers are those sold under the trade name Teflon(copyright) by E. I. DuPont de Nemours Co., Inc. of Wilmington, Del. Examples of these types of coatings used in cookware and bakeware are disclosed in U.S. Pat. No. 5,863,608 to Swisher et al., which is also incorporated by reference herein. However, amorphous fluorocarbon polymer coated vessels for cooking and baking tend to lose their non-stick quality over time as the coating can be easily scratched, abraded or otherwise marred or worn away because of its generally soft texture and as the fluorocarbon polymer loses its lubricity over time. These fluorocarbon polymer coatings are oftentimes referred to as xe2x80x9cPTFExe2x80x9d coatings.
I have found that metal surfaces can have improved non-stick or reduced friction properties if they are made harder. In the case of multi-layer clad cookware, where softer heat conducting metals such as aluminum and/or copper are used as the core metal, which is then laminated on both sides with a harder metal such as stainless steel, the cook surface is generally softer than might be attained without a laminated metal. Metals, such as stainless steel, can be hardened by austenitizing and tempering at very high temperatures. When a softer metal core, such as copper or aluminum, is used in a laminate, the ideal temperatures for tempering or austenitizing stainless steel would melt the lower melting point core metal. Such a heat treatment would result in warping or deformation of the multi-layer, composite cooking utensil. Therefore, optimum hardening temperatures for tempering and austenitizing steel cannot be used with composite metals containing aluminum and/or copper. This results in a less hard and less than ideal cook surface.
There remains an unmet need for cookware and bakeware that have excellent and uniform heat transfer properties while providing a reliable and durable non-stick cooking surface, be it a bare metal surface, a PTFE type coating or other coated surfaces such as vapor deposited ceramic nitrides.
The present invention is directed to cookware, bakeware and other food preparation surfaces such as grills and griddles that have increased surface hardness and improved stick resistance and longer life stick resistant coatings, in the case of coated cookware. The instant cookware, bakeware and other food preparation surfaces are made from metals, either bare or precoated with a non-stick coating, which have been cryogenically processed to increase their hardness.
The present invention is further directed to a method of making cookware, bakeware and like food preparation surfaces, hereinafter collectively referred to as xe2x80x9ccookwarexe2x80x9d. The method includes the steps of forming a clad sheet of metal with a core of aluminum or copper laminated on one or both sides with stainless steel, aluminum or the like. The clad sheet is then cryogenically tempered at less than xe2x88x92100xc2x0 F. and more preferably less than xe2x88x92275xc2x0 F. Ideally, the cookware, in either a bare or coated condition, is subjected to a cyclic cryogenic treatment wherein the temperature is cycled between about xe2x88x92100xc2x0 F. to xe2x88x92280xc2x0 F. or xe2x88x92300xc2x0 F. or lower for a period of time and then subjected to a temper treatment at about +280xc2x0 F. to +300xc2x0 F.